Easy, while the sheer quantity of privately owned guns have gone up (Thanks Obama), the number of households that own guns has actually decreased significantly over the past 40 years.

Basically, the people that buy guns buy lots of guns so this graph doesn't really show much other than paranoid gun nuts buy lots of guns. So while the number of guns out there has seen a steady increase, the number of people that actually have access to them has decreased just as fast or faster.

It seems to me that the decline in household ownership correlates more with the declining gun homicide rate rather than the total number of guns purchased. Less than 1/3rd of households own firearms, which is historically low.